Generally, the present invention is concerned with the treatment of wastewaters which contain biodegradable solids. Such wastewater may emanate from sewage collection systems, oil refineries, coke plants, paper making plants, canneries, food processing plants and the like. The treatment of these organic dissolved and suspended materials is typically accomplished by a process commonly classified as an aerobic treatment process. Removal of the organic material by these processes is accomplished by two general mechanisms. First, impurities are adsorbed or adsorbed at the interface between the associated biomass and the wastewater. Second, the biomass decomposes these organics through oxidation. The resulting increased biomass or sludge consisting of accumulated micro-organisms is generally separated from the organically stabilized liquid. Most of the biomass is generally returned to the process to continue the process and the excess sludges are periodically removed from the system.
In conventional biological treatment systems, the major components are typically an aeration basin and a clarifier tank. The aeration tank may be rectangular or circular and contain means for continually circulating the mixed liquor (suspended solids and waste liquid) within the tank with the addition of oxygen or air to promote micro-organism growth. The aeration basin may also be generally oval in shape and define a trough-like channel having bottom and spaced upstanding side walls for retaining and circulating the mixed liquor in a continuous substantially closed flow path, which is often referred to as an "oxidation ditch". The mixed liquor is continuously circulated by means of rotating brushes, discs, turbines or the like, at a flow velocity to maintain the solids in suspension. Additional air or oxygen may also be added to the circulating mixed liquor to promote micro-organism growth.
In both the aeration basin system and the oxidation ditch system, a clarifier is required to separate suspended solids from the mixed liquor and to withdraw clarified liquid. The clarifier is typically a separate unit located adjacent the aeration tank or oxidation ditch, and serves as settling tank for separating suspended solids from the mixed liquor by gravity. The clarified liquid may be disposed of or reused, while the settled biomass remains in the clarifier, from which it may be disposed of as waste sludge, or recycled to the aeration tank or oxidation ditch to maintain the proper balance between organic loading and biological microbial mass solids in the mixed liquor. The separate clarifiers typically require pumping means to transmit mixed liquor from the aeration basin or oxidation ditch to the clarifier and/or pumping means to transmit the settled biomass from the clarifier back into the aeration basin or oxidation ditch. The separate clarifiers also require slow speed scraper drive and scrapers to remove the settled sludge therefrom. It has been found that the use of such separate clarifiers not only require significant installation and material costs together with land space for the clarifier installation, but also a significant expenditure of energy resources to remove the settled sludge from the clarifier, as well as to move the sludge between the aeration basin or oxidation ditch and the clarifier.
In an effort to resolve these problems, in recent years there have been various system proposals to provide internal clarifier devices positioned within the oxidation ditch. These clarifier devices are commonly referred to as intrachannel clarifiers. Examples of such intrachannel clarifiers are disclosed in U.S. Pat. Nos. 4,303,516, 4,383,922, and 4,446,018.
While such proposed systems have eliminated or reduced some of the above enumerated problems associated with systems employing separate clarifiers and oxidation ditches, they are not without drawbacks of their own. For example, certain of these proposed systems incorporate intrachannel clarifier basins supported within the oxidation ditch above the bottom of the ditch or aeration section, which substantially fill the cross section of the aeration channel in order to increase the velocity in the channel to create a lower head. This increases the head that the circulation device, i.e. the brush aerator, must work against and thus lowers velocity and oxygen transfer as well as increasing horsepower. Essentially, the intrachannel clarifiers operate more or less as conventional gravity settling clarifiers. They typically require intricate bottom baffling arrangements to return the settled solids to the aeration channel through large open areas in the bottom of the clarifier basin. There is a tendency for the turbulent flow in the aeration channel to divert some of the flow upwardly through these large open areas and thereby disturb the settling flow. Further, it is necessary to carefully control the flow velocity in the aeration channel to prevent withdrawing excess solids and liquid through the large number and large area of the openings in the bottom of the clarifier basin.
The aeration and mixing section of the oxidation ditch requires the inducement of oxygen into solution in the mixed liquor. The oxygen must be supplied in a volume required by the BOD load and flow rate. The flow rate through the oxidation ditch may vary as much as 10 to 1 in small systems and 2 to 1 in large systems. On domestic sewage and many industrial wastes the BOD load in the mixed liquor flow path remains relatively constant per unit volume. To satisfy this oxygen demand, this variation has heretofore been typically handled by supplying oxygen at a rate sufficient to handle the peak load through the system and letting the excess oxygen build up the dissolved oxygen level at reduced flows. The dissolved oxygen built up at low flows reduces the amount of oxygen transferred and is thereby inefficient in operation and uses excess amounts of energy. It has been proposed to provide long mechanical weirs within the oxidation ditch, requiring intricate sealing and manual operators, to adjust the liquid level in the oxidation ditch and thereby control the oxygen transfer rate. Placement of such weirs in the oxidation ditch frequently results in a wave action in the oxidation ditch and, as the waves splash over the weir, a dewatering effect results causing a drop in aeration of the mixed liquor. This effect may happen at various levels in the oxidation ditch and often requires other aeration to avoid this problem.
In recently issued U.S. Pat. No. 4,487,692, a wastewater treatment system is proposed which includes a clarifier mounted on a wall defining a closed loop oxidation ditch. The system utilizes brush aerators for moving the mixed liquor through a flow path in the ditch and to introduce oxygen into the mixed liquor. The clarifier is narrow and elongated in relation to the flow path. The clarifier has closed side walls and end walls and an open lower end to permit mixed liquor flow from the oxidation ditch to enter therethrough. The mixed liquor flows vertically upward from the lower end to the upper end at a sufficiently low velocity to permit organic solids entering the clarifier to exit therefrom under the influence of gravity through the same open lower end through which the mixed liquor flow enters. The lower end is provided with vertically extending baffles disposed across the flow path in much the same manner as the prior intrachannel clarifiers. As with the prior intrachannel clarifiers, the upward flow therethrough slows down settling and reduces the efficiency of the clarifier. The low velocity flow through the clarifier is controlled by a plurality of means provided along the length of the clarifier for withdrawing treated or clear liquid from the upper end of the clarifier. This means includes a plate or a weir having a number of openings formed therein at a common elevation below the liquid level in the oxidation ditch. The effluent launder which receives the clear liquid is provided with a mechanically adjustable weir which may be manually adjusted to regulate the liquid level in the oxidation ditch and thereby control the rate of oxygen transfer within the system. The weir must be manually adjusted to vary the liquid level in the oxidation ditch and does not automatically adjust such level in proportion to the influent flow into the oxidation ditch.